1. Field of the Invention
The present invention relates to a process for producing croconic acid (i.e., 4,5-dihydroxy-4- cyclopentene-1,2,3-trione) or a salt thereof.
2. Description of the Related Art
Croconic acid or a salt thereof is useful as a dye, a medicine, an agricultural chemical and the like, or as an intermediate thereof (see, for example, JP-A-2001-294785, JP-A-2001-117201 and JP-A-5-155145). As a related synthetic method for such a compound, for example, it is known that croconic acid is present in a reaction mixture formed by the oxidation of inositol with fuming nitric acid (see Journal of the American Chemical Society Vol. 52, p. 2483 (1930)), or a metal salt of croconic acid is obtained by performing oxidative decarboxylation of tetrahydroxybenzoquinone obtained through two steps from chloranil (see Chemical World, Vol. 3, p 139 (1999)). However, there has been a demand for the development of an inexpensive and simple process for producing croconic acid or a salt thereof.